Camera tube target including n-type semiconductor having higher concentration of deep donors than shallow donors



Sept. 24, 1968 Filed Feb. 7, 1967 DONORS THAN SHALLOW DONORS O 2 Shets-Sheet 1 LIGHT 20 I/ I m FIG. 7

1 I 77 ll/ //7 lNl/ENTORS VG J. A. MORTON ATTORNEY Sept. 24, 1968 D. KAHNG ET AL 3,403,278

CAMERA TUBE TARGET INCLUDING N-TYPE SEMICONDUCTOR HAVING HIGHER CONCENTRATION OF DEEP DONORS THAN SHALLOW DONORS v Filed Feb. 7 1967 2 Sheets-Sheet 2 AQUJNS" United States Patent 3,403,278 CAMERA TUBE TARGET INCLUDING N-TYPE SEMICONDUCTOR HAVING HIGHER CON- CENTRATION 0F DEEP DONORS THAN SHALLOW DONORS Dawon Kahng, Bridgewater Township, Somerset, and Jack A. Morton, South Branch, N.J., assignors to Bell Telephone Laboratories, Incorporated, Murray Hill, N.J., a corporation of New York Filed Feb. 7, 1967, Ser. No. 614,457 13 Claims. (Cl. 313-65) ABSTRACT OF THE DISCLOSURE In one embodiment, a television camera tube includes a target structure comprising a flat semiconductor with a grid electrode bonded to a target surface and a transparent insulative film overlayed by a transparent conductor bonded to a light admitting surface. The semiconductor contains a relatively high concentration of deep donor centers which are ionized by incoming light to form a charge density pattern which, by electron beam scanning, is converted to a video output signal. Electrons are transmitted from the semiconductor by the grid conductor. In another embodiment, the semiconductor is bonded to a flat conductor by a junction contact and light is admitted on the target surface.

Background of the invention Of the television camera tubes presently available, the vidicon offers the best advantages of low cost and simplicity of adjustment. The vidicon includes a flat photoconductor, one surface of which is periodically scanned with an electron beam, the opposite surface being coated with a transparent conductive film and being exposed to incoming light. The conductive film is biased positively with respect to the tube cathode that projects the electron beam.

Light impingements on various parts of the photoconductor excite electrons from the valence band to the conduction band, thereby reducing localized resistivity as a function of light intensity. Electrons that have been deposited on the photoconductor target surface by the electron beam therefore leak across each photoconductor increment to the conductive film at a rate determined by the average light intensity to which that increment has been subjected. Hence, during the period taken to scan the entire target surface, or frame period, a voltage pattern is formed on the target surface. As the electron beam again scans the target surface, the charge it deposits varies with time in accordance with the stored photoconductor voltage pattern and therefore with the integrated illumination of successive photoconductor increments. Due to capacitive coupling, differences of electron deposition are manifested by current variations in the conductive film, which are taken as the output video signal.

It has been recognized that vidicon photoconductors must meet conflicting conditions: they must have a low enough resistivity in response to light to be reasonably sensitive, while having a sufliciently high transverse resistivity across the target surface to maintain the voltage pattern during the frame period. Substantial transverse currents due to insufficient transverse resistivity smear the image read out by the beam. The most satisfactory material presently available for fulfilling these requirements is antimony trisulfide, which, unfortunately, inherently presents several other problems. It loses its photoconductivity when subjected to high temperatures, which precludes vidicon fabrication by conventional vacuum tube techniques; the entire tube cannot be outgassed by high temperature treatment, which in turn compromises its reliability. Further, antimony trisulfide has a limited useful life expectancy, and it is susceptible to severe damage by overexposure both to light and to electron beam energy.

Summary of the invention Our invention circumvents these problems by using a semiconductive mechanism other than photoconductivity for storing energy representative of an image. As a result, the need for forming and maintaining a target surface voltage pattern is eliminated, as is the need for a photoconductor for producing long lifetime carriers, which alters and increases the choice of materials that can be used. Specifically, materials of long life, durability, and compatibility to vacuum tube fabrication technology can be used.

In an illustrative embodiment, our invention comprises a vidicon-type camera tube having the usual cathode and beam forming and deflecting apparatus. Opposite the cathode is a flat n-type semiconductor, one surface of which is the target surface. A transparent insulator is sandwiched between the other side of the semiconductor and a transparent conductive film. A conductor bonded to the periphery of the target surface is biased at a small positive voltage with respect to the conductive film.

The semiconductor contains a high proportion of deep donors; that is, donors that cannot be thermally ionized at the temperature of operation. Incoming light through the transparent conductive and insulative films, however, excites electrons of these deep donors into the conduction band after which they are swept away from the semiconductor by the space-charge field through the target surface conductor, leaving behind positive deep donor ions. These ions are substantially immobile and so their relative concentration is indicative of localized input light intensity. Electrons from the electron beam are attracted to and recombine with the deep donor ions thus reducing the localized voltage across the insulator. Current increments are generated in the conductive film in response to the voltage changes which are taken as the video output signal. Since the energy stored in the device is defined by relative concentrations of immobile deep donor ions, the transverse resistivity problem described before is less severe than in conventional vidicons, and semiconductor materials can be used that are more durable and offer fewer fabrication problems than vidicon photoconductors. More importantly, the material requirements are altered because the mechanism of operation is fundamentally different from that of conventional vidicons.

Drawing description These and other objects, features, embodiments, and advantages of our invention will be understood from a consideration of the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic view of a television camera tube in accordance with one embodiment of the invention;

FIG. 2 is an enlarged schematic view of the target structure of the television camera tube of FIG. 1;

FIG. 3 is a view taken along lines 3-3 of FIG. \2;

FIG. 4 is a typical energy band diagram of the target structure of FIG. 2;

FIG. 5 is a typical energy band diagram of the target structure of FIG. 2 under conditions of light energy excitation;

FIG. 6 is a typical energy band diagram of part of the semiconductor of FIG. 2 illustrating various excitation mechanisms; and

FIG. 7 is a schematic view of a television camera tube in accordance with another embodiment of the invention.

Detailed description Referring now to FIG. 1, there is shown, as an illustrative embodiment of the invention, a vidicon-type tele vision camera tube having a cathode 11 for forming and projecting an'electron beam toward a target structure 12. The tube includes conventional electron beam alignment, accelerating and focusing structure, and conventional apparatus for deflecting the beam in a line and frame sequence. Light is incident on the target structure from the side opposite the electron beam; this light is appropriately focused on the target structure to define an image to be recorded and transmitted. The purpose of the tube, like that of a conventional vidicon, is to convert incoming light energy to electrical energy, store the electrical energy, and release or read out the stored energy in the form of a continuous video signal as indicated by the output arrow.

Referring to FIGS. 2 and 3, our novel target structure 12 comprises a semiconductor 14 one surface of which defines the electron beam target surface 15, with the other side being bonded to a transparent insulative film 16. A transparent conductive film 17 is bonded to the insulative film 16 and to a transparent glass substrate 18 that forms part of the tube envelope as illustrated in FIG. 1. A grid conductor 20 including an array of parallel conductor strips 21 is bonded to the target surface and is biased at a positive voletge V with respect to the conductive film 17. The grid conductor forms an equipotential plane over the entire target surface. An insulator coating 22 overlaying the grid conductor shields the grid conductor from the beam. The dimensions shown have been distorted for purposes of clarity.

In accordance with the invention, the semiconductor is appropriately treated to be n-type; that is, it contains a preponderance of donor, rather than acceptor, centers. Further, it contains a higher concentration of deep donor centers than shallow donor centers. Deep donors are donor centers that cannot be ionized by thermal agitation at the temperature to which it is subjected, while shallow donors are those that can be thermally ionized. The significance of this distinction and the function of the target structure can be better appreciated from the energy band diagrams of FIGS. 4 through 6.

Referring to FIG. 4, the portions M, I, S, respectively represent the energy band distributions in the transparent conductor film 17, the insulator film 16-, and the semiconductor 14. The letters C represents the lower boundaries of the conduction bands, V the upper boundaries of the valence bands, and F, the Fermi level. The subscripts 1 indicate band configuration with no bias voltage across the structure, the subscript 2 denoting their response to the applied bias voltage V of FIG. 2.

Under a zero bias condition, the Fermi level F is common to the three materials. F is, of course, closer to the semiconductor conduction band edge C than to the valence band edge V because the semiconductor is n-type. When the bias voltage V (shown in FIG. 2) is applied, the semiconductor conduction and valence band edges bend to the positions shown by C and V the energy difference between F and F being equal to V Electrons that are thermally excited to the conduction band from shallow donor atoms are swept out by the electric field and transmitted away by the target surface grid conductor 20, leaving behind shallow donor positive ions which define a depletion region, or spacecharge region 5C Since the films 16 and 17 are transparent, light impinges directly on the semiconductor and alters its energy band configuration as shown in FIG. 5. The incoming light excites electrons from deep donors thereby creating a larger positive ion concentration than had existed before. The electrons excited from the deep donors, like those excited from the shallow donors, are transmitted away from the semiconductor by the target surface grid conductor 20.

This has the effect of shortening the space-charge 4.. region 50 and reducing the voltage drop across thesemiconductor 5; it also increases-. the localized voltage'drop across the insulator I. Considering ?the' insulator.to be a dielectric between parallelcapacitor plates, -thepincreased voltage across the insulator is the equivalent of a'capacitor charging voltage. a

Various excitation mechanisms in the semiconductor are illustrated in FIG: 6, in'which solid dots represent electrons, encircled positive symbols represent positive ions, and positive symbols that are not encircled represent holes. The shallow donor has an energy level sufiicientlyclose to-theaconductionband tov permit thermal excitation of .an electron 25, thereby leaving a shallow donor ion 26. Incoming light may ionize deep donors either by direct or indirect --light excitation. Direct excitation results fromthe impingement of a photon on a deep donor which raises an electron 27 to the conduction band and produces a positive deep donor ion 28. Indirect excitation is caused by the excitation of an electron 29 from the valence band to the conduction band, which pro duces a hole 31 in the valence band. An electron 'from a deep donor atom then recombines with the hole 31, thus producing a deep donor ion 32. Relative deep donor ion concentrationsare a function of the numberof incoming photons and therefore-the relative intensities of the in-- coming light at difierentlocations along the interface of the semiconductor with the insulator. The resulting charge density distribution along the interface is maintained be. cause deep donor ions are substantially immobile.

A' video outputsignal representative of the chargedensity distribution is generated when the target surface. is scanned by the electron beam. Referring to FIG.'1, notice that the resistor R is part of a circuit defined by the battery and the target structure, with the output signal being a function of the current through R Whereas in a conventional vidicon the target is analogous to. a succession of resistors having difierent resistances, the target structure of our device is analogous to a succession of capacitors having different charges. ,Difierent charges stored on opposite sides of each insulator increment are, of course, indicative of the average light intensity on that increment during one frame period.

As the beam impinges on each target increment, beam electrons recombine with the deep donor ions and thereby discharge the incremental capacitor. A current increment then flows through R in response to the discharge and a voltage across the output terminals is -generated.- The successive output voltages therefore depend on the charge on successive capacitor increments so that the time varying output voltage is a function .of the average spatial light intensity distribution on the target surface over one.

frame period. The spacing of the parallel strips 21 of the grid conductor 20 is chosen such that the electrons are swept out before they can recombine withpositive ions in the adjacent regions. After scanning, the, capacitor increments are again charged due to incoming light; this charging current mayor may not interfere with the derived signal voltage depending on the specific apparatus being used. Since the chargingcurrent must flow to the.

target surface grid-conductor-ZO, the current from the battery to conductor 20 maybe sampled and subtracted from the output signal to eliminate-this problem.. Ap-.

propriate circuitry for implementing this function is within the skill of a worker in the art.

For proper operation, the deep donondensity in the semiconductor should be more than about 10 per cubic centimeter and preferably, in the range of 10 per cubic centimeter, while the shallow donor concentration should be less than about 10 per cubic centimeter. ,.In order to permit indirect excitation, the band gap (BG of FIG. 6), or the energy separation between valence andconduction bands, should .be 'smaller than the photon energy of light to be rec0rded;-In anycase, the energy separation between the deep donors and the-conduction band must not be greater than the photon energy. On the other hand,

if the band gap is too small, deep donor doping is diflicult. These various requirements can be met by using for the semiconductor material one of the IIVI compounds such as cadmium sulfide and cadmium selenide or a III-V componud such as gallium arsenide. These materials can be doped to high deep donor concentrations by conventional techniques and, as is known, their deep donor densities can be determined in various ways. Moreover, the carrier mobility and dielectric relaxation time of cadmium sulfide, cadmium selenide, or gallium arsenide are appropriate for lateral electron transit times that are large with respect to the recombination time.

Typical dimensions of the target structure may be as follows: length and width, /2 x /z inch; semiconductor thickness, 1-10 microns; insulative film thickness, .1 micron, conductive film thickness, .1 micron. The parallel strips 21 of the grid conductor 20 may each be about microns wide, separated from each other by about 100 microns, and covered by a suitable insulator of several tenths of a micron thick and about 30 microns wide. Conventional thin film techniques, particularly vapor deposition, can be used for making the target structure to these dimensions. For an application that tolerates less resolution, single crystal material, with or without epitaxially grown layers, can also be used.

The target surface grid conductor 20 may be biased at about +1 volt with respect to the conductive film 17 and at about +5 to +30 volts with respect to the cathode 11. With these parameters and with an electron beam density typically used in vidicon tubes, the diffusion gradient and the space-charge forces on impinging beam electrons will be appropriate to cause sufiicient diffusion into the semiconductor to insure substantially complete recombination with the ionized deep donors in accordance with the invention. The spacings of parallel strips 21 are sufficiently small to avoid impairment of resolution due to spurious electron recombination. Other spacings can be used depending on the semiconductor characteristics or the desired resolution, or, as described below, the strips can be eliminated if secondary emission electron removal is used.

If the shallow donor concentration and the electron mobility are such that the transverse or lateral conductance is high, resolution may be impaired by transverse spreading of beam electrons. It can be shown that appropriate resolutions for vidicon operation are assured by additionally making the resistivity of the semiconductor to be in the range of 10 to 3x10 ohm-centimeters. The lower limit of this range is sufficient for giving a fundamental output frequency of 10 c.p.s. and the upper limit gives a sulficient dielectric relaxation time to permit electrons to be swept out by the grid conductor within a frame time of of a second. The lower limit may be made still lower if the recombination can be made to occur in less than 10- sec.

The parallel strips 21 may be eliminated if provision is made to extract that portion of the electrons injected into the semiconductor by the electron beam which is in excess of the number of electrons required for deep donor recombination. This can be achieved, for instance, by modulating the electron beam energy in time about the average value; at beam energies lower than this average value electrons are injected, and at higher energies electrons are extracted due to secondary emission. The beam may be velocity modulated by driving an appropriate electron gun grid with modulating energy from a source 23 shown in FIG. 1. For operation corresponding to a fundamental output frequency of 10 c.p.s., the modulating frequency should be (1 to 10) X10 c.p.s. Design of the relevant tube parameters to give a secondary emission ratio in excess of one in response to high energy electrons is within the ordinary skill of a worker in the art.

As is apparent from the above description, the purpose of the insulative film 16 is to prevent any current flow between the semiconductor 14 and the conductive film 17; it is an electronic barrier. Alternatively, an electronic barrier between the metal and semiconductor can be made by coating the semiconductor directly onto the metal such as to make a junction contact which is substantially insulative under reverse bias conditions. Also, because the semiconductor 14 of FIG. 1 is thin, there is nothing in principle that requires incoming light to im pinge on the side of the semiconductor Opposite the target surface; it could be directed at the target surface. These two alternative aspects of the invention are illustrated by the embodiment of FIG. 7.

The television camera tube 40 of FIG. 7 has a target structure 41 comprising an n-type semiconductor 42 bonded by a junction contact to a conductor 43. A grid conductor 44 is bonded to the target surface of the semiconductor in the same manner as the target surface conductor of FIG. 3. The tube 40 is an iconoscope-type camera tube having a cathode 45 for forming an electron beam that is deflected in a manner well known in the art to scan the target surface in a line and frame sequence. The cathode 45 is appropriately displaced so that light can be focused directly on the target surface scanned by the electron beam.

The mechanism of storage and read-out is essentially the same as that of the device of FIG. 1. Deep donors in the semiconductor are ionized by incoming light, with the scanning beam generating an output voltage across R as a function of the resulting charge density distribu tion in the semiconductor. Electrons excited to the conduction band of the semiconductor from deep and shallow donors and from the valence band are swept out by the target surface conductor 44. The junction between the conductor 43 and semiconductor 42 establishes a suflicient barrier to preclude any current flow across it.

Although large area junction contacts can be made and have been made by vapor deposition of semiconductor material onto metal to a thickness of only a few microns, it should be emphasized that great care is required during fabrication for forming a junction or barrier layer having a uniformly high energy difference between the metal and semiconductor to be substantially and uniformly insulative under reverse bias conditions. Localized imperfections on the metal surface being coated, foreign impurities, or irregular deposition can result in interface regions capable of high rates of carrier generation and thus a poor junction contact. As is known, the edges of the metal to semiconductor junction should be masked to minimize the edge generation problem.

Because of the fabrication problems inherent in thin film large area semiconductor-metal junction contacts, and the acknowledged advantages of vidicon operation over iconoscope operation, the embodiment of FIG. 1 is presently strongly preferred over the embodiment of FIG. 7. Future technological developments may, however, lead to a preference for the FIG. 7 embodiment, the use of a metal-insulator-semiconductor target structure of the type shown in FIG. 1 in the iconoscope tube of FIG. 7, or the use of a metal-semiconductor junction contact target structure of the type shown in FIG. 7 in a vidicon tube. In the latter modification the metal conductor would, of course, be thin and transparent.

It is to be understood that the embodiments shown and described are intended merely to be illustrative of the inventive concept of our invention. Various other modifications and embodiments may be made by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In an electronic storage tube, the combination comprising:

a semiconductor one surface of which defines a target surface;

a substantially transparent conductive film;

a substantially transparent insulative film included be- 7 tween and bonded to the conductive film and the semiconductor;

said semiconductor being n-type and containing a higher concentration of deep donors than shallow donors;

said conductive and insulative films defining a light transmission path, thereby enabling incoming light to ionize semiconductor deep donors;

a conductor on the target surface for conducting from the semiconductor electrons released by the ionization of donor particles, whereby the ionized deep donors establish a relatively stable space charge region having localized densities which are a function of corresponding localized light input;

means for scanning the target surface with an electron beam;

and means comprising an output conductor connected to the conductive film for deriving an electrical signal which is a function of successive localized space charge densities of the semiconductor as it is scanned by the electron beam.

2. The combination of claim 1 wherein:

the deep donor density of the semiconductor is more than about 10 per cubic centimeter and the shallow donor density is less than about 10 per cubic centimeter.

3. The combination of claim 1 further comprising:

means comprising the target surface conductor for establishing over substantially the entire target surface an equipotential plane of higher positive voltage than the D-C voltage of the conductive film.

4. The combination of claim 1 further comprising:

means comprising the target surface conductor for establishing over substantially the entire target surface an equipotential plane of higher positive voltage than the D-C voltage of the conductive film;

and wherein the target surface conductor comprises an array of parallel conductive strips extending across the target surface.

5. The combination of claim 4 further comprising:

a cathode for forming and projecting the electron beam;

and means for maintaining the conductive film at a higher positive D-C voltage than the cathode.

6. In an electronic storage tube, the combination comprising:

an n-type semiconductor having first and second opposite surfaces;

said semiconductor containing a higher concentration of deep donors than shallow donors;

a fiat conductor substantially coextensive with, and in close proximity to, the second semiconductor surface;

means for establishing an electronic barrier between the semiconductor and the flat conductor of sufficient magnitude to preclude substantially any current flow therebetween;

means for focusing an optical image on the semiconductor, thereby ionizing semiconductor deep donors as a function of localized light intensity;

a target surface conductor on the first semiconductor surface for conducting from the semiconductor electrons released by the ionization of donor particles, whereby ionized deep donors establish a relatively stable space-cha'rge region having localized densities which are a function of corresponding localized light input intensities;

means for scanning the first surface of the semiconductor with an electron'beam;

and means comprising an output conductor connected to the conductive film for deriving an electrical signal which is a function of successive localized spacecharge densities of the semiconductor as it is scanned by the electron beam.

7. The combination of claim 6 wherein:

the barrier establishing means comprises a flat insulator bonded to the flat conductor and the second semiconductor surface.

8. The combination of claim 6 wherein:

the fiat conductor is bonded to the second semiconductor surface by a junction contact, the-junction con tact constituting the electronic barrier.

9. The combination of claim 6 further comprising:

means for varying the velocity of the electron beam between a value at which the secondary emission ratio of the target structure is greater thanl. and a value.

at which the secondary emission ratio of the target structure is smaller than 1.

10. The combination of claim 9 wherein:

the frequency of said variation is approximately in the range of 10 to 10" cycles per second.

11. The combination of claim 6 further comprising:

means comprising the target surface conductor for establishing over substantially the entire first surface an equal potential plane of higher positive voltage than the D-C voltage of the conductive film.

12. The combination of claim 9 wherein:

the target surface conductor-comprises an array of parallel conductive strips overlaid with insulator material and each separated 'by a distance of about References Cited g UNITED STATES PATENTS- 2,890,359 6/1959 Heijne et al. 313- 3,268,764 8/1966 Simms 31365 X 3,289,024 11/1966 De H'aan et al. 313 -65 ROBERT SEGAL, Primary Examiner. 

